1. Field of the Invention
This invention relates to a method for treating colored liquid such as a solution containing dyes, waste water from the dyeing industry and waste ink from ink-jet printers.
2. Related Background Art
Dyes and waste water from the dyeing industry are generally treated by means of a combined process of aggregation/precipitation or pressure flotation and activated sludge treatment to reduce chromaticity, BOD and COD thereof. However, the decolorizing effect of such a process is far from satisfactory in spite of the large scale facilities and long processing time, hence the waste water treated with the process still contains dyes to a considerable extent, and may damage the scenic beauty of rivers and the productivity of living things in the river water as dyes in the water will block out the light transmission. In recent years, the above process is further combined with electrolysis, a tertiary treatment using active carbon or an oxidizing agent or dilution with water to improve the decolorizing effect. However, a process combining aggregation/precipitation and the use of active carbon for adsorbing dyes for the purpose of removal/decolorization of dyes is costly and not very effective particularly in a low dye concentration range because dyes can hardly be adsorbed and removed in such a range.
One of the other methods attracting attention in recent years and providing a subject for technological research and development is an electrolysis/oxidation process utilizing electrochemical reaction. It is a process of decolorizing dyes in waste water in an electrolytic cell through electrolysis/oxidation. For example, xe2x80x9cDyeing Researchxe2x80x9d, vol. 40, No. 2 (1996) p.19 and Japanese patent Applications Laid-Open Nos. 5-115879 and 8-281271 describe techniques using such a process. However, all the proposed techniques require a large treatment cost and hence there is still a strong demand for a novel method and a novel apparatus that can decolorize dyes effectively and efficiently.
Other known techniques for decolorizing colored liquid include the use of an apparatus for decolorizing dyes by means of ozone obtained by irradiating oxygen with ultraviolet rays (Japanese Patent Application Laid-Open No. 9-239383) and that of an apparatus for decolorizing colored liquid by adding an additive thereto and irradiating it with ultraviolet rays. However, both of them require UV light of high energy with a wavelength lower than 300 nm, so that a material that does not transmit ultraviolet light such as glass or plastic cannot be used for such an apparatus, requiring a vessel made of an unusual material such as quartz and a special light source for UV light.
As a result of intensive research for solving the above identified problems, the inventors of the present invention have found that a dye solution can be decolorized quickly by contacting the dye solution with functional water obtained by electrolysis of water containing an electrolyte, under irradiation with light.
The present invention is based on the novel findings and an object of the present invention is to provide a simple method for efficiently and stably decolorizing colored liquid to a low concentration range.
Another object of the invention is to provide a method of decolorizing colored liquid reliably in a short period of time.
According to a first aspect of the present invention, there is provided a method for decolorizing colored liquid comprising the steps of: providing functional water by electrolysis of water containing an electrolyte; and bringing a colored liquid into contact with the functional water under irradiation with light to reduce chromaticity of the colored liquid.
According to another aspect of the invention, there is provided an apparatus for decolorizing colored liquid, comprising a container provided with a pair of electrodes, a power source for applying a voltage to the electrodes, a means for supplying water containing an electrolyte to the container, a light irradiation means and a means for supplying colored liquid to the container.
According to still another aspect of the invention, there is also provided an apparatus for decolorizing colored liquid comprising a decomposition treatment container, a means for supplying functional water obtained by electrolysis of water containing an electrolyte to the decomposition treatment container, a means for supplying a colored liquid to be decomposed to the decomposition treatment container and a means for irradiating the decomposition treatment container with light.
The reason why various organic compounds are acceleratingly decomposed when irradiated with light in the presence of functional water is not clear. However, the inventors presume that the water produced by electrolysis of water containing an electrolyte such as sodium chloride typically contains hypochlorous acid or hypochlorite ions, which then produce chlorine radicals, hydroxyl group radicals or superoxides by the action of light to promote the above decomposition reaction.
Japanese Patent Application Laid-Open No. 8-281271 discloses a process for decomposing a dye contained in dyeing water in an electrolytic vessel by means of hypochlorous acid and/or hypochlorite ions generated by electrolysis. xe2x80x9cMIZUSHORI GIJUTSU (Water Treatment Technology)xe2x80x9d, vol. 37, No. 5 (1996), p. 33 discloses a process for treating dyeing disposal water, by using an electrochemical reaction. That is, it discloses a process for decomposing dyes in the dyeing waste by using an indirect electrolysis, generating an oxidizing agent by electrolysis, indirectly decomposing the dyes by the oxidizing effect thereof. It also discloses that an example of the oxidizing agent includes hypochlorous acid, which is generated through a reaction of chlorine gas produced at the anode and hydroxy ions produced at the cathode when sodium chloride is added to the disposal water and subjected to electrolysis. However, neither of the journal article nor the patent document cited above describes or suggests that the colored liquid is decolorized efficiently by bringing the colored liquid into contact with functional water under irradiation with light.